Zero power plasmonic microelectromechanical device

ABSTRACT

A zero-power plasmonic microelectromechanical system (MEMS) device is capable of specifically sensing electromagnetic radiation and performing signal processing operations. Such devices are highly sensitive relays that consume no more than 10 nW of power, utilizing the energy in detected electromagnetic radiation to detect and discriminate a target without the need of any additional power source. The devices can continuously monitor an environment and wake up an electronic circuit upon detection of a specific trigger signature of electromagnetic radiation, such as vehicular exhaust, gunfire, an explosion, a fire, a human or animal, and a variety of sources of radiation from the ultraviolet to visible light, to infrared, to terahertz radiation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/753,059, filed 15 Feb. 2018 as the U.S. national phase ofPCT/US2016/048083, filed 22 Aug. 2016, which claims the priority of U.S.Provisional Application No. 62/207,545 filed 20 Aug. 2015 and entitled“Zero-Power Plasmonic Microelectromechanical Infrared Digitizer”. Eachof the aforementioned applications is hereby incorporated by referencein its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was developed with financial support from Grant No.HR0011-15-2-0048 from the Defense Advanced Research Projects Agency. TheU.S. Government has certain rights in the invention.

BACKGROUND

Current state-of-the-art sensors use active electronics and requirecontinuous power. However, there is a need for microelectromechanicaldevices such as sensors, switches, relays, and digitizers that operateunder low power, zero power, or near zero power standby conditions,which allows them to be deployed for long-term or remote operationwithout the need for battery replacement or a power source.

SUMMARY OF THE INVENTION

The invention provides a zero-power plasmonic microelectromechanicalsystem (MEMS) device capable of specifically sensing electromagneticradiation and performing signal processing operations. Devices of theinvention serve as highly sensitive relays, consuming no more than 10 nWof power, and have a low false positive rate. The device utilize theenergy in detected electromagnetic radiation to detect and discriminatea target without the need of any additional power source. The devicescan continuously monitor an environment and wake up an electroniccircuit upon detection of a specific trigger signature ofelectromagnetic radiation. Devices of the invention can sense thepresence or proximity of vehicular exhaust, gunfire, an explosion, afire, a human or animal, and a variety of sources of radiation from theultraviolet to visible light, to infrared, to terahertz radiation.Moreover, sensors using the devices can detect specific combinations ofsuch radiation to identify unique targets.

One aspect of the invention is a zero-power plasmonicmicroelectromechanical relay. The relay includes a substrate and a firstcantilever disposed on or within the substrate. The first cantileverincludes a head, an inner pair of temperature-sensitive bimaterial legs,and an outer pair of temperature-sensitive bimaterial legs. The innerpair of legs are attached to opposite sides of the head, while the outerpair of legs are attached to the substrate and disposed adjacent to theinner pair of legs, forming first and second sets of inner and outerlegs. The first and second sets of legs are disposed symmetrically onopposite sides of the head. The relay further includes a first thermalisolation region connecting the inner and outer legs of the first set oflegs, and a second thermal isolation region connecting the inner andouter legs of the second set of legs. The relay also includes a topmetal layer attached to the head which serves as a contact element. Theposition of the contact element determines whether the relay is in an ONor OFF state. The relay further includes a source electrical contactdisposed on the surface of the substrate and a drain electrical contactdisposed on the surface of the substrate, the drain contact separatedfrom the source contact by a gap. In an embodiment, the head includes aplasmonic absorber that absorbs electromagnetic radiation within aspectral band selected for detection of a target. The absorption ofelectromagnetic radiation within the spectral band causes movement ofthe head, causing the contact element to move toward the source and/ordrain contacts; when sufficient radiation within the selected spectralband impinges on the absorber, a conductive path is provided betweensource and drain contacts, and the associated circuit is closed.

An important feature of the relay is that the bimaterial legs eachcomprise a stack of at least two materials having different thermalexpansion coefficients. This, and the design of the device, allow thelegs to provide compensation for ambient temperature changes as well asfor residual stress in the device from the fabrication process.

The relay is sensitive to electromagnetic radiation and can be switchedfrom OFF to ON by as little as 100 nW of radiation in a vacuum and aslittle as 10 μW in air. The threshold can be adjusted to higher levelsas desired, such as at least 200 nW, 500 nW, or 1, 2, 5, or 10 μW in avacuum or at least 50, 100, 200, or 500 μW, or 1, 2, 3, 5, or 10 mW inair. The relay can undergo repeated cycling, such as 1 or more cycles,10 or more, 100 or more, 500 or more, 1000 or more, 10000 or more 100000or more, or 1000000 or more cycles without degradation of performance.

Another aspect of the invention is a device containing one or morerelays as described above. The device can be, for example an exhaust gasdetector, a living organism detector, a proximity sensor for a heatsource or an organism, an infrared detector, a visible light detector, acolor sensor, a spectrograph, or an electro-optical switch.

The invention can be further summarized by the following list ofembodiments.

1. A zero-power plasmonic microelectromechanical relay comprising:

-   -   a substrate;    -   a first cantilever disposed on the substrate, the first        cantilever comprising a head, an inner pair of        temperature-sensitive bimaterial legs, and an outer pair of        temperature-sensitive bimaterial legs, the inner pair of legs        attached to opposite sides of the head, the outer pair of legs        attached to a surface of the substrate and disposed adjacent to        the inner pair of legs forming first and second sets of inner        and outer legs, the first and second sets of legs disposed        symmetrically on opposite sides of the head;    -   a first thermal isolation region connecting the inner and outer        legs of the first set of legs, and a second thermal isolation        region connecting the inner and outer legs of the second set of        legs;    -   a top metal layer attached to the head which serves as a contact        element, wherein position of the contact element determines        whether the relay is in an ON or OFF state;    -   a source electrical contact disposed on the surface of the        substrate; and    -   a drain electrical contact disposed on the surface of the        substrate, the drain contact separated from the source contact        by a gap;    -   wherein the head comprises a bottom metallic layer and a top        insulating layer;    -   wherein the head further comprises a plasmonic absorber that        absorbs electromagnetic radiation within a spectral band        selected for detection, and the absorption of such        electromagnetic radiation causes the contact element to move        toward the source and/or drain contacts;    -   wherein the bimaterial legs each comprise a stack of at least        two materials having different thermal expansion coefficients,        the legs providing compensation for ambient temperature changes;        and    -   wherein absorption of said electromagnetic radiation by said        plasmonic absorber causes the contact element to form an        electrical connection between the source and drain contacts.        2. The relay of embodiment 1, further comprising a second        cantilever disposed on the substrate adjacent to the first        cantilever and with mirror symmetry to the first cantilever, the        second cantilever comprising a head, an inner pair of        temperature-sensitive bimaterial legs, and an outer pair of        temperature-sensitive bimaterial legs, the inner pair of legs        attached to opposite sides of the head, the outer pair of legs        attached to a surface of the substrate and disposed adjacent to        the inner pair of legs forming first and second sets of inner        and outer legs, the first and second sets of legs disposed        symmetrically on opposite sides of the head, the second        cantilever further comprising a first thermal isolation region        connecting the inner and outer legs of the first set of legs,        and a second thermal isolation region connecting the inner and        outer legs of the second set of legs; wherein the head of the        second cantilever comprises a bottom metallic layer and a top        insulating layer and does not comprise a plasmonic absorber;        wherein the bimaterial legs of the second cantilever each        comprise a bottom insulating layer and a top conductive layer,        the legs providing compensation for ambient temperature changes;        and wherein the bimaterial legs of the first cantilever and the        bimaterial legs of the second cantilever provide similar        temperature compensation.        3. The relay of embodiment 2, wherein the second cantilever head        comprises a top metallic layer disposed on the insulating layer        opposite the bottom metallic layer, wherein the top metallic        layer functions as a broadband reflector.        4. The relay of embodiment 2, wherein the second cantilever head        comprises an absorber that absorbs electromagnetic radiation        over a different spectral band than that absorbed by the        plasmonic absorber of the first cantilever head.        5. The relay of any of the preceding embodiments, wherein said        ON state is characterized by existence of an electrical        connection between the source and drain contacts, and said OFF        state is characterized by the existence of no electrical        connection between the source and drain contacts, and wherein        the relay consumes less than 10 nanowatts of power in the OFF        state.        6. The relay of embodiment 5 that exists in the ON state in the        presence of said electromagnetic radiation and exists in the OFF        state in the absence of said electromagnetic radiation, and is        capable of repeatedly switching between said ON and OFF states        solely based on the presence or absence of said electromagnetic        radiation.        7. The relay of embodiment 6 that is capable of at least 500        cycles of switching between the ON and OFF states.        8. The relay of any of the preceding embodiments that can be        switched from the OFF state to the ON state by less than 1 μW of        said electromagnetic radiation in a vacuum and less than 3 mW of        said electromagnetic radiation in air.        9. The relay of embodiment 5 that exists in the OFF state in the        presence of said electromagnetic radiation and exists in the ON        state in the absence of said electromagnetic radiation, and is        capable of repeatedly switching between said OFF and ON states        solely based on the presence or absence of said electromagnetic        radiation.        10. The relay of any of the preceding embodiments, wherein said        electromagnetic radiation is in the range from about 100 nm to        about 3 mm in wavelength.        11. The relay of any of the preceding embodiments, wherein said        electromagnetic radiation is infrared radiation.        12. The relay of any of the preceding embodiments, wherein said        spectral band has a bandwidth in the range from about 0.5% to        about 100% of a peak absorption wavelength.        13. The relay of any of the preceding embodiments, wherein said        spectral band has a bandwidth of about 8% of a peak absorption        wavelength.        14. The relay of any of the preceding embodiments, wherein said        contact element comprises a bowl-shaped structure that forms        said electrical connection between the source and drain contacts        in an ON state.        15. The relay of any of the preceding embodiments, wherein the        first cantilever head comprises a metamaterial plate having        microscale length and width and nanoscale or microscale        thickness, and wherein the plate comprises a metallic base        layer, an insulating layer disposed on the metallic base layer,        and one or more patterned metallic structures disposed on the        insulating layer and designed to absorb electromagnetic        radiation within said spectral band.        16. A device comprising a plurality of relays according to any        of the preceding embodiments, wherein at least two of said        relays absorb and are switched by different spectral bands of        electromagnetic radiation.        17. The device of embodiment 16, wherein at least some of said        plurality of relays are connected in series to detect        simultaneous presence of a plurality of different spectral bands        of electromagnetic radiation.        18. The device of embodiment 16 that utilizes a combination of        serial and parallel connections of said plurality of relays.        19. The device of any of embodiments 16-18 that is capable of        identifying a source of said electromagnetic radiation based on        a spectral fingerprint of said radiation.        20. The device of any of embodiments 16-19, wherein one or more        of said plurality of relays detects out-of-band electromagnetic        radiation not produced by said source.        21. The device of embodiment 20, wherein the out-of-band        electromagnetic radiation is black body radiation.        22. A device comprising one or more relays according to any of        the preceding embodiments, wherein the device is selected from        an exhaust gas detector, a living organism detector, a proximity        sensor, an infrared detector, a visible light detector, a color        sensor, a spectrograph, and an electro-optical switch.        23. A zero-power electrothermal microelectromechanical relay        comprising:    -   a substrate;    -   a first cantilever disposed on the substrate, the first        cantilever comprising a head, an inner pair of        temperature-sensitive bimaterial legs, and an outer pair of        temperature-sensitive bimaterial legs, the inner pair of legs        attached to opposite sides of the head, the outer pair of legs        attached to a surface of the substrate and disposed adjacent to        the inner pair of legs forming first and second sets of inner        and outer legs, the first and second sets of legs disposed        symmetrically on opposite sides of the head;    -   a first thermal isolation region connecting the inner and outer        legs of the first set of legs, and a second thermal isolation        region connecting the inner and outer legs of the second set of        legs;    -   a top metal layer attached to the head which serves as a contact        element, wherein position of the contact element determines        whether the relay is in an ON or OFF state;    -   a source electrical contact disposed on the surface of the        substrate; and    -   a drain electrical contact disposed on the surface of the        substrate, the drain contact separated from the source contact        by a gap;    -   wherein the head comprises a bottom metallic layer and a top        insulating layer;    -   wherein the head further comprises an electric heater that is        capable of heating the head and causing the contact element to        move toward the source and/or drain contacts;    -   wherein the bimaterial legs each comprise a bottom insulating        layer and a top metallic layer, the legs providing compensation        for ambient temperature changes; and    -   wherein a threshold amount of heat provided by said electric        heater causes the contact element to form an electrical        connection between the source and drain contacts.        24. The relay of embodiment 23, further comprising a second        cantilever disposed on the substrate adjacent to the first        cantilever and with mirror symmetry to the first cantilever, the        second cantilever comprising a head, an inner pair of bimaterial        legs, and an outer pair of bimaterial legs, the inner pair of        legs attached to opposite sides of the head, the outer pair of        legs attached to a surface of the substrate and disposed        adjacent to the inner pair of legs forming first and second sets        of inner and outer legs, the first and second sets of legs        disposed symmetrically on opposite sides of the head, the second        cantilever further comprising a first thermal isolation region        connecting the inner and outer legs of the first set of legs,        and a second thermal isolation region connecting the inner and        outer legs of the second set of legs; wherein the head comprises        a bottom metallic layer, a top insulating layer, and an electric        heater that is capable of heating the head and causing the        contact element to move toward the source and/or drain contacts;        and wherein the bimaterial legs of the second cantilever each        comprise a stack of two or more layers having different thermal        expansion coefficients, the legs providing compensation for        ambient temperature changes; and wherein the bimaterial legs of        the first cantilever and the bimaterial legs of the second        cantilever provide similar temperature compensation.        25. A microelectromechanical device comprising a bowl-shaped        structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an embodiment of a plasmonicmicroelectromechanical infrared digitizer of the invention. Thezoomed-in views at the bottom of the figure show details of theplasmonically-enhanced MEMS relay.

FIG. 2 is a top view illustration of a plasmonic MEMS relay device.

FIGS. 3A-3D show the results of finite element model (FEM) simulationsfor a plasmonic MEMS relay device.

FIGS. 4A-4B show the results of 3D FEM simulations of the effect oftemperature on cantilever head deflection.

FIGS. 5A-5B depict an embodiment of a plasmonic MEMS relay deviceintegrated with piezoelectric unimorph actuators.

FIGS. 6A-6E show modeling of a plasmonic metasurface. FIG. 6A presentsan electromagnetic circuit model of a plasmonic metasurface. FIG. 6Bshows measured and simulated absorption of a plasmonic metasurface witha=1.64 μm and b=313 nm. FIG. 6C shows a schematic representation of aunit cell of a plasmonic metasurface. FIG. 6D shows an IR absorbingmetasurface of a PMR of the invention.

FIG. 6E shows a finite integration technique (FIT) simulated absorptionof the plasmonic absorber of FIG. 6D.

FIGS. 7A-7C show schematic representations of different switch contactelement configurations.

FIG. 8A shows a logic topology for the reduction of interference bynon-target objects involving series connections of PMRs. FIG. 8B shows alogic topology using a combination of series and parallel connections ofPMRs.

FIG. 9 shows a top view of a design for an IR-absorbing PMR.

FIGS. 10A-10C show simulation results for temperature compensation inthe PMR design of FIG. 9 .

FIG. 11A shows simulation results for stress and stiffness in twocontact element designs. FIG. 11B is an SEM image of the improveddesign.

FIG. 12A shows a comparison of IR absorbance for plasmonic absorbersusing square vs. cross-shaped patterns. FIG. 12B shows IR absorptionspectra for plasmonic absorbers tuned to absorption of NO and CO gasses.

FIG. 13 shows an SEM image of a PMR device.

FIG. 14 shows results of varying IR input power.

FIG. 15 shows results of repeated cycling of a PMR device.

FIG. 16A shows an SEM image of an electrothermal microelectromechanicalrelay. FIG. 16B shows switching characteristics of the relay of FIG.16A.

FIGS. 17A-17F illustrate steps of a process for preparing amicroelectromechanical relay device including a plasmonic absorber.

FIG. 18 shows overlapping masks for SiO₂ and Al layers of bimateriallegs of a PMR device.

FIG. 19 shows steps of a photoresist plug shaping process for trenchplanarization in preparing a PMR device.

FIGS. 20A-20F show steps of preparing a contact tip for a PMR device.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have developed a zero-power plasmonicmicroelectromechanical system (MEMS) device that combines sensing,signal processing, and comparator functionalities into a singlemicrosystem capable of producing a quantized output bit or serving as arelay in the presence of unique spectral signatures while consuming nomore than 10 nW of power and maintaining a low false alarm rate. Thedevice utilizes the energy in detected electromagnetic radiation todetect and discriminate a target without the need of any additionalpower source. The device is capable of continuously and passivelymonitoring an environment, and then waking up an electronic circuit upondetection of a specific trigger signature of electromagnetic radiation.

In an embodiment of the device, a plasmonically-enhanced MEMS relay(PMR) is selectively activated by impinging radiation at a specificspectral wavelength, and a conducting channel is created between sourceand drain metal contacts and a metal contact element. The radiation canbe, for example, infrared (IR) radiation emitted from a target that iswarmer than its environment. A thin plasmonic metasurface is used toform an IR-sensitive head of the PMR, which allows the whole device tobehave as a spectrally selective IR ultrathin absorber (i.e., only thewavelength selected by the metasurface can activate the PMR). The deviceis sensitive, responding to an IR emission of less than 100 nW, andspectrally selective, having a full width at half maximum of <10%. Ithas a fast response time of <100 ms to close contacts upon exposure toIR radiation. The structure is completely immune to ambient temperaturechanges and residual stress thanks to an intrinsic compensation scheme.The PMRs integrate body-biased thin-film AIN piezoelectric unimorphactuators to achieve threshold tuning and complementary deviceoperation. The device rejects background interference from other warmobjects by using a passive logic circuit to connect the system batteryto the load only when exposed to the target IR signature. The devicethus combines near zero power consumption with high sensitivity andrejection of false positives.

A device of the invention has many possible uses. For example, it can beused as an unattended ground sensor that lasts for years and identifiedremote threats, such as vehicular pollution, gunfire, explosion, naturalfire, and the like. It can be used as a miniaturized and zero-powercombat identification trigger that remains covert unless illuminated bya friendly laser target designator. It can be used in wearable sensorsthat constantly monitor radiation levels of surroundings with zero-powerconsumption. Finally, a device of the invention can be employed as azero-power sensor node for the Internet of Things.

In one embodiment of the device, an array of plasmonically-enhanced MEMSrelays (PMRs) are connected to implement a passive logic circuit. Thedevice separates the system battery from an output load capacitor. EachPMR, according to its design, is selectively triggered by impingingradiation at a specific spectral wavelength. When the anticipatedspectral signature is absent, at ambient background conditions, thesystem battery is disconnected from the output load, resulting in adigital output voltage of <10 mV. However, when exposed to the targetspectral signature (e.g., the exhaust plume of a vehicle), a specificcombination of PMRs is activated, according to the implemented logic,connecting the system battery to the load capacitor which is charged toa voltage value of ≥1V, indicating positive detection of the target.

An infrared digitizer of the invention is shown schematically in FIG. 1. The device exploits the energy in heated gas molecules to detect anddiscriminate the presence of an exhaust plume of interest, whilerejecting background interference from other warm objects, without theneed of any additional power source, which directly translates intonear-zero standby power consumption. These unique features have beenachieved by MEMS, plasmonic, and microsystem design.

A core element of devices and systems of the invention is a MEMS relay,an embodiment of which is shown in FIG. 2 . In some embodiments, theMEMS relay is a plasmonically activated MEMS relay (PMR), which isactivated by impinging electromagnetic radiation within a specificspectral wavelength band. The absorbed power is used to create aconducting channel between two bottom contacting metals (source 51 anddrain 53) and a top metal tip or layer 41. PMR 10 consists of twosymmetric released cantilevers facing each other: first cantilever 20and second cantilever 30. Each cantilever is composed of a head (21,31); an inner, thermally sensitive pair of bimaterial legs (23, 33)(bottom insulating later (71) and top conductive layer (73)); an outertemperature and stress compensating pair of bimaterial legs (25, 35)connected to the substrate (24); and a pair of thermal isolation regions(27) between the bimaterial legs. The head of the second cantilever iscomposed of a metal-insulator-metal structure that reflects anyimpinging IR radiation (relatively thick top metal layer acts as amirror). Source and drain contacts (bottom contacting metals of the PMR)are defined in the top metal layer of the head of the second cantileverand are electrically connected to the respective terminals on thesubstrate through the legs of the second cantilever structure (FIG. 1 ).The head of the first cantilever carries the electrically floating metaltip (top contacting metal, or contact element, of the PMR). The firstcantilever head is composed of a metal-insulator-metal structure inwhich the top metal layer (65) is patterned to form an array ofplasmonic nanostructures that enable strong absorption of IR radiationin the sub-wavelength structure (plasmonic absorber 67) ((see FIG. 1 andlower portion of FIG. 2 for cross-section (e.g. metamaterial plate 60having bottom metallic layer or metallic base layer 61, top insulatinglayer 63, patterned metallic structures 65)).

When an IR beam impinges on the device from the top, it is selectivelyabsorbed by the plasmonic head of the first cantilever, leading to alarge and fast temperature increase of the freestanding micromechanicalstructure (i.e., head and inner leg portions) up to the two thermalisolation regions, where the high thermal resistivity of the insulatormaterial (e.g., SiO₂) links the inner and outer legs and prevents heattransfer to the outer legs and the substrate, see FIGS. 1 and 2 ). Suchan IR-induced temperature rise results in a downward bending of thefirst cantilever's thermally sensitive pair of bimaterial legs due tothe in-plane thermal stress caused by the large difference in thethermal expansion coefficients of the two materials forming each leg(e.g., 2 μm thick SiO₂ as the insulating layer and 500 nm thick Al asthe metallic layer). This thermally-induced bending translates intovertical displacement of the first cantilever head. On the other side,the first cantilever's outer compensating pair of bimaterial legs aswell as the entire second cantilever do not experience any temperaturevariation upon exposure to IR radiation (FIG. 3 ). Therefore, the airgap (55) separating the bottom drain/source contacts patterned on thehead of the second cantilever from the top metal tip on the head of thefirst cantilever is directly controlled by the impinging IR radiationthrough a plasmonically-enhanced thermomechanical coupling. When thepower of the IR radiation exceeds the designed threshold, the PMR isactivated, and the top metal tip is brought into contact, shorting thesource and drain contacts allowing current to flow from the systembattery to the load. Despite the intrinsically high sensitivity of thePMR to IR-induced heat, the structure is completely immune to ambienttemperature changes; both the inner and outer pairs of the legssupporting the two cantilevers bend in the same direction, thusminimizing any ambient temperature induced motion of the cantileverheads. Similarly, any mechanical bending of the cantilever legs due toresidual stresses associated with the fabrication process is alsominimized. Any residual ambient temperature and/or stress-induceddeflections of the two cantilevers are compensated by the symmetry ofthe structure; because both the cantilevers deflect in the same fashion,the designed gap dimension is preserved.

For the PMR embodiment shown in FIG. 2 , the thermal resistance of thedevice was estimated using finite element model (FEM) simulations,considering the conductive and radiative heat transfer mechanisms(simulating the vacuum case). The FEM simulation estimated a thermalresistance of ˜2×10⁶° K/W for the structure (FIG. 3A), which isdetermined by the parallel combination of conductive (˜4×10⁶° K/W) andradiative (˜4×10⁶° K/W) thermal resistances. The FEM simulations alsoshowed that a relatively large displacement of the PMR plasmonic head isachieved for smaller amounts of absorbed IR power (sensitivity ˜1.2nm/nW, FIGS. 3B, 3C). The structure spring constant was also estimatedby FEM simulation and found to be ˜25 mN/m. Therefore, an absorbed IRpower of ˜500 nW would be sufficient to close a ˜500 nm gap with arestoring force at the contact of ˜12.5 nN, which is large enough toovercome adhesion forces and re-open the contacts (reset the PMR) oncethe IR radiation is no longer present. Based on these FEM simulationresults, the contact force was also calculated as a function of thedistance between the plume (at two different temperatures) and the PMR,including atmospheric absorption effects and assuming a 150 mm diameterconcave mirror is employed to focus the light onto the PMR (FIG. 3D).

The PMR is intrinsically immune to ambient temperature variations ifperfectly symmetric cantilever heads and legs are employed (FIG. 4A).However, the plasmonic structure patterned on the head of the firstcantilever and the Al layer (used for electrical routing) deposited ontop of the thermal isolation regions of the second cantilever can inprinciple deteriorate such a balanced condition. This effect wasevaluated by 3D FEM simulation using COMSOL showing a gap sensitivity toambient temperature variations of ˜4.3 nm/° C. for such imperfectlymatched cantilevers (FIG. 4B). Although such a small temperaturesensitivity might be considered tolerable (since a relatively large gapof ˜500 nm is employed), the deflection mismatch between the two headscan potentially be canceled by patterning a highly reflective thermalmatching layer (e.g., a gold patch) on the head of the second cantileverto match the deflection of the head of the first cantilever. Forexample, FEM simulations showed that a highly reflective 118 μm×118 μmAu patch, patterned on the head of the second cantilever is sufficientto reduce the gap sensitivity to temperature to ˜0.02 nm/° C. (FIG. 4B).Such intrinsic immunity to ambient temperature and residual stress ishighly beneficial for the implementation of normally open (NO) PMRs withdesign determined IR threshold values (designed gap is preserved afterfabrication). Nevertheless, by properly engineering an asymmetry in thestructure (i.e., by acting on the material and the geometry of thethermal matching layer patterned on the head of the second cantilever,either at the layout level and/or by trimming after fabrication) it ispossible to fabricate high performance normally closed (NC) PMRsrequiring only a small amount of restoring force, hence a low amount ofabsorbed IR power, to overcome the stress asymmetry in the beams (inaddition to surface adhesion) and switch to an open state.

Threshold tuning and complementary device operation, using NO and NCPMRs, can be further facilitated by the introduction of body-biasedthin-film aluminum nitride (AlN) piezoelectric unimorph actuators in thedesign of the structure (FIGS. 5A-5B), providing a multitude ofadvantages for the implementation of a sophisticated logic for improvedIR signature discrimination. In an embodiment of the PMR integrated withpiezoelectric unimorph actuators, thin layers of AlN (500 nm) and metal(molybdenum, 100 nm, to be used as the bottom electrode) are added overthe inner and outer legs of the structure (sandwiched between the SiO₂structural layer and the Al top metal electrode). Since the thermalisolation of the PMR is primarily determined by the dimensions ofthermal isolation regions (conductive heat transfer) and the cantileverhead area (radiative heat transfer), a high thermal resistance (˜2×10⁶°K/W) is preserved despite the introduction of the AlN and Mo layers ontop of the PMR inner and outer legs. FEM simulations showed that: (i) adownward bending of the inner piezoelectric cantilever legs of ˜450 nmis achieved for a head temperature variation of 1° K. (corresponding to˜500 nW of IR power absorbed in the cantilever head); and (ii) adisplacement of ˜150 nm is achieved at the tip of the innerpiezoelectric leg (head displacement) when a 1 V bias is applied to theouter piezoelectric leg. Although the added piezoelectric layer makesthe legs ˜25% less sensitive to the IR induced temperature variation,trimming and reconfiguration of the PMR IR sensitivity and threshold canbe achieved by controlling the size of the air gap with the appliedbias; body-biasing of the piezoelectric actuator causes a pre-bending ofthe beams, enabling tuning of the device IR threshold by modulating thesize of the initial contact gap, and providing the capability ofconfiguring the device to be normally closed (i.e., by closing thecontact with the applied body-bias and configuring the IR sensitivecantilever to deflect downwards to open the contact upon absorption ofIR power). Despite the use of a body-bias, a near-zero leakage currentcan be achieved due to the excellent dielectric properties of the AIN,maintaining the overall power budget of the device at <10 nW.

The accuracy of such a threshold adjustment is fundamentally limited bythe voltage stability of the battery employed to provide the bias, sincea voltage regulator will not be employed, in order to preserve a 10 nWpower budget. Commercially available 1.5 V lithium batteries(non-rechargeable primary cell) lose about 0.7% of their capacity peryear when stored at room temperature (from the handbook of EnergizerL91), which corresponds to a nominal voltage reduction of ˜1.5 mV/yearwhen the discharge current is close to zero. Furthermore, the capacityof such batteries is very stable within a wide range of ambienttemperature variations (−40° C. to 60° C.) when the discharge current issmall (<25 mA). Therefore, the effect of short-term temperature changeon the output voltage of such batteries can be considered negligible. Inthe worst case scenario, if the batteries are stored at 40° C. for 10years, they will lose ˜30% of their capacity, and the open-circuitoutput voltage will decrease by ˜4%. Therefore, the initial contact gapof the body-biased piezoelectric PMR can be designed to tolerate biasvariations as large as ˜5%. The contact size and initial gap of the PMRalso can be designed to prevent the PMR from pulling in due toelectrostatic forces. In order to avoid parasitic electrostaticsource/drain actuation of the plasmonic head, the source/drain contactarea and initial gap can be designed to guarantee a pull-in voltagelarger than the maximum voltage applied between drain and source of thePMR (i.e., system battery voltage). For example, for the preliminary PMRdesign proposed here, a total source/drain contact area, A<10 μm² wouldguarantee a pull-in voltage, V_(PI)>8 V, significantly larger than thetypical OFF state source-drain voltage value (˜1V). The NO and NC PMRdesigns can be experimentally verified, and the design parametersincluding contact materials, contact force, surface adhesion, air gap,length of the legs, and thickness of the material stack can beaccurately modeled and optimized to achieve an IR threshold ≤500 nW, asub-threshold swing ≤10 pW/dec (expected ˜9 orders of magnitude changein the drain-source current between ON and OFF states) and a number ofswitching cycles >10.

A fundamental requirement for the correct functionality of theplasmonically-enhanced MEMS relay described above is that the incidentradiation at the target spectral wavelength is efficiently absorbed inthe head of the first cantilever while the incident radiation at all theother frequencies is reflected. Although different kinds of IR absorberscan be integrated on top of the head of the cantilever, the resultingrelatively thick material stack can negatively affect the mechanical andthermal response of the relay. Preferably, a thin plasmonic metasurface(˜2 μm thick) is used to form the IR sensitive head of the PMR, whichallows the whole device to behave as a spectrally selective IR ultrathinabsorber. The metasurface can be composed of a thin dielectric (i.e. the2 μm thick SiO₂ structural layer of the PMR) sandwiched between a bottommetal plate (i.e. 100 nm Pt) and a top metal layer (i.e. 50 nm Au)patterned to form an array of plasmonic nanostructures that enablestrong and spectrally selective absorption of IR radiation in thesub-wavelength structure. Such a plasmonic metasurface can be modeledusing a transmission-line approach (FIG. 6A). While a conventionallongitudinal resonance would lead to a significant thickness, severelyaffecting the mechanical and thermal response of the resonator, in apreferred design the plasmonic metasurface is patterned on top of thegrounded ultra-thin dielectric to have a large capacitive surfacereactance. X_(s)=−1/(ωC_(s)). Stacked on top of a grounded slab, thedominant resonance is achieved when X_(s)=−Z₀ tan(βd), where Z₀ and βare the characteristic impedance and propagation constants of thedielectric. Therefore, by tailoring the surface reactance of theplasmonic metasurface to be largely capacitive, it is possible to inducean ultrathin Fabry-Perot-like resonance in such a sub-wavelengthstructure. The inventors have experimentally demonstrated a 600 nm thickpiezoelectric plasmonic metasurface (composed of a 500 nm AlN filmsandwiched between a 100 nm bottom Pt plate and a 50 nm top metal layerpatterned to form an array of plasmonic nanostructures) capable ofabsorbing ˜80% of the impinging IR radiation for an optimized spectralband centered at ˜8.8 μm (FWHM˜1.5 μm) (FIG. 6A). Multiple metasurfacegeometries can be utilized in order to optimize absorption coefficientand FWHM at the spectral wavelengths of interest for plume IR signaturedetection. A plasmonic head design (compatible with the PMR discussedabove), tuned to the emission band of CO₂ is shown in FIGS. 6C,6D.Full-wave simulations using the commercial software CST indicated that:(i) nearly 100% absorption can be achieved despite the sub-wavelengththickness of the structure; (ii) a narrow FWHM of ˜5% can be attained;and (iii) by lithographically changing the geometrical dimension of thearray of plasmonic nanostructures, absorption peaks at different IRspectral wavelengths can be readily obtained (FIG. 6E). Many metasurfacestructures are known which can serve as plasmonic absorbers. See, forexample, Watts, C. M., Liu, X., & Padilla, W. J. (2012). Metamaterialelectromagnetic wave absorbers. Advanced Materials, 24(23).(doi.org/10.1002/adma.201200674) and Cui, Y., et al. (2014). Plasmonicand metamaterial structures as electromagnetic absorbers. Laser andPhotonics Reviews, 8(4), 495-520. (doi.org/10.1002/Ipor.201400026).

The PMRs of the invention have a low IR threshold (≤100 nW) and very lowcontact forces, both when in contact and on retraction from contact,requiring contacts with low adhesion. Therefore, the adhesion force isan important contact design parameter. Conventional contact mechanicspredict that to minimize adhesion for a given material, the contactpoint should have a small radius using a high hardness material with asmall surface energy.

Theoretically, a larger radius contact (trending toward flat) exhibitsmore adhesion in a predictable manner. For nanometer radius contacts,the adhesion force can be in the nano-Newton range (nN). Forconventional contacts, the overall design size of the contact is notvery important, because it is the nanostructure that determines thecontact radius, and therefore the contact area. Excess adhesion from hotswitching is minimal because the voltage (less than a few volts) andcurrent are limited by the circuit design, eliminating the possibilityof arcing and minimizing the amount of heating and material transfer.With regard to resistance, even in a somewhat extreme case, where thecontact radius is ˜3 nm and the resistivity ˜20 μΩ-m, the resistancewould still only be ˜3000Ω, which is adequate for the purposes of thePMRs of the present invention. Contact contamination is low as thesystem operates at low-pressure for thermal isolation.

Adhesion in MEMS/NEMS contacts covers an extremely wide range from less1 nN (atomic force microscope) to tens or even hundreds of μN(low-resistance MEMS switches). To achieve ultra-low adhesion contacts,both the surface energy and the plastic deformation (at moderate contactpressures) are reduced compared to conventional MEMS contacts. Theproblem in a conventional contact is that the contact area increaseswith contact force (both applied force and adhesion force) due toplastic deformation of the contact material. In a conventionalconducting material contact, a purely elastic contact is only observedfor hard materials at low forces. In the case of at least some of thedevices of the present invention, these conditions are likely to hold,and the contact adhesion is not expected to increase greatly overthousands or millions of contacts. Therefore, contacts can be preparedwith small-radius asperities fabricated from hard refractory materialssuch as tungsten, ruthenium, or ruthenium oxide.

Several different switch designs are suitable for use in a PMR of theinvention. FIGS. 7A-7C show three such designs. FIG. 7A shows a“1-contact” type, in which contact is made between the top metal contactelement and one bottom metal contact. FIG. 7B shows a 2-contact type, inwhich two 1-contact type switches are used, connected by a conductivemember. FIG. 7C shows a “2-contact joint” design in which a top contactelement shorts two closely-spaced bottom contact terminals (source anddrain). The 1-contact and the 2-contact designs are different mainly inthe way the current flows through the switch. The 1-contact typerequires a conductive path that travels from the source (V+) through thetop metal layer (Al) in the bimaterial outer leg of the first cantilever(i.e., absorber side structures), the isolation region, the inner legsof the second cantilever, across the switch, through the inner legs ofthe second cantilever (the reflector side structures), the isolationregion, and lastly the outer legs of the second cantilever to the drain(V−). Thus the current has to pass through half of each side. All testsin the Examples were done using the 1-contact configuration. The2-contact configurations are different in that the current flows mostlythrough the second cantilever. This would mean that the absorbing sidedoes not require the use of a metal on the thermal isolation region,which increases the thermal resistance and improves sensitivity. The2-contact configurations also allow for further applications requiringlow ON resistance, since the first cantilever is intrinsicallycompensating and the second cantilever can be entirely replaced by justtwo closely spaced fixed terminals, completely avoiding the longresistive path of the second cantilever.

The sensitivity (minimum power required to close the contact gap) can betuned primarily by changing the length of the bimaterial legs. Longerlegs improve sensitivity but reduce the stiffness required to overcomeadhesion and turn OFF. Aluminum and SiO₂ can be used for the bimateriallegs to maximize sensitivity, since they have a large difference inthermal expansion coefficients and are CMOS process compatible. Otheroptions to improve sensitivity are to reduce contact gap, to optimizethe thickness ratio of the bimaterials, and to improve the thermalresistance of the thermal isolation regions by either using differentmaterials or optimizing dimensions.

The stiffness of the contact tip is an important parameter forovercoming the adhesion of the contact. The stiffness of the relay hasto be larger than the stiffness of the cantilever (which is about 0.05N/m). The stiffness is a function of the length of the contact tip andmaterial thickness, so the contact tip can be engineered to anystiffness. However, the use of bowl-shaped designs for the contact tipenables the maximum possible stiffness. By reducing the tip length by4×, a 64× increase is obtained due to the inverse cubic dependence ofstiffness on length. In general, the stiffness of the contact tip canrange from about 0.05 N/m to about 1000 N/m, or from about 40 N/m to1000s of N/m.

The false alarm rate can be minimized through the implementation of PMRlogic topologies. Using such integrated passive hardware logic, the PMRsof the invention are capable of performing a spectral analysis ofincoming IR radiation and detecting the presence of, for example, anexhaust plume of interest while rejecting background interference fromother targets with different spectra. A logic topology, such as the oneshown in FIG. 8A, employing a combination of normally open (NO)“in-band” PMRs (deigned to absorb IR wavelengths emitted by the target)and normally closed (NC) “out-of-band” PMRs (designed to absorb IRwavelengths that are not emitted by the target), can be implemented. Ifall the NO in-band PMRs and at least one of the NC out-of-band PMRs areactivated, then the device will output a low logic bit ‘0’. If insteadall the NO in-band PMRs but none of the NC out-of-band PMRs areactivated, then the device will output a high logic bit ‘1’. Thanks tothe minimal effect of the drain-to-source voltage on the actuation ofeach PMR (parasitic electrostatic force is minimized as explainedabove), the switching transients of an individual element will notelectrically trigger adjacent devices.

Another logic topology involves a combination of series and parallelconnections between PMRs. This can be employed to eliminate signals fromhot objects that are not targets. Since a hot object emits across alarge spectrum unlike a discretely emitting gas, there are certainwavelength regions where there are no gaseous emission bands and theseare referred to as “out-of-band” (OOB) emissions. By connecting oneterminal of PMRs sensitive to these out-of-band wavelengths to theoutput node (at the end of the series “in-band”/gas-sensitive PMRs) andthe other to ground, it can be ensured that whenever the out-of-bandsignatures are present, the output is pulled down to the groundpotential. This is represented in FIG. 8B.

Instead of connecting the OOB PMRs in parallel as described above, theycan be intentionally made normally closed (NC) and connected in seriesto the in-band PMRs. The normally closed PMRs can be fabricated byintentionally having a small gap between the contacts such that adhesionforces dominate over the restoring force from the stiffness of thestructure. With the right gap it is possible to engineer them to open onexposure to IR (note that here, the side with the bottom contact—thesecond cantilever—needs to have the plasmonic absorber too).

Another implementation of series-only topology is to detect bothout-of-band (OOB) and in-band absorbers on a single PMR. For example, anOOB absorber can be placed on the head of the second cantilever whilehaving an in-band absorber on the head of the first cantilever. When theOOB wavelengths are present, the OOB absorber head bends down,increasing the contact gap to effectively prevent closing of the switch.If both the OOB and in-band signals are present, both sides benddownwards, maintaining the contact gap and preventing the switch fromclosing. This allows for a small overall package while minimizing falsedetections. Other low power applications for PMRs:

PMR devices of the invention also can be used for visible lightdetection and color sensing. Since the device is entirely based ontemperature rise of the head, it can be adapted to incorporate any typeof absorber (broadband, black gold, Fabry-Perot layer stacks, and thelike) on the head of the first cantilever. It also can be used to detectvisible wavelengths of light, for example by using simple broadbandabsorbers with color filters for zero power color sensing. A device wastested in air for detection of visible light at 405 nm. The device useda plain gold layer (no plasmonic pattern) on the head of the firstcantilever which intrinsically has ˜60% absorption in the violet region.An ON threshold of ˜3.5 mW was measured.

The devices also can be used as zero power PIR (passive IR) sensors.Such devices can be used in situations requiring low powerhuman/animal/object detection in remote areas. Current PIR sensorstypically use circuits that always consume a finite amount of power,requiring periodic battery replacements or constant power supply. Due tothe high degree of temperature immunity, they can be used in virtuallyany kind of location, even outer space. The devices also can be used forzero power proximity sensing in smartphones, which currently use anear-IR LED and photodiode just above the screen to turn off the screenwhen a user brings the phone close to the ear when talking. The LEDsconsume power and drain the already taxed battery. The PMRs of theinvention can perform proximity detection without requiring an IR sourceapart from the heat of the human body. A similar logic topology toprevent false detections such as due to IR from the sun can be easilyimplemented. Use of the PMRs of the invention also can reduce thefootprint of presently used circuits for IR detection, since the use ofan LED and detector requires relatively large drive and sense circuits.

PMRs of the invention also can be employed in high reliabilityelectro-optical switching and in fiber-optics-based communication giventheir high reliability and sensitivity. The ability to incorporatemultiple narrow band wavelength sensing in a single package couldtremendously improve speeds at low frequencies, since extra informationin form of different wavelengths can be sent through the same path anddetected by all PMRs in parallel with high immunity to cross talkbetween PMRs.

EXAMPLES Example 1. Temperature Compensation in a Plasmonic MEMS Relay

A folded bimetal symmetric PMR structure was fabricated according to thedesign shown in FIG. 9 . The device was based on a classic intrinsicallycompensating structure, wherein the absorptive head (91) with topcontact and the reflecting head (93) with bottom contact have negligiblerelative movement due to ambient temperature change or stress. The thinisolation region contained the heat absorbed in the absorbing headwithin the inner pair of bimaterial legs. It should be noted that eitherside is intrinsically compensating to an extent, but to improve thecompensation further and to add functionality, a mirrored structure wasused.

From the simulations and subsequent profilometer measurements as shownin FIGS. 10A-10C, the excellent compensation afforded by the structurewas confirmed, with only ˜50 nm offset between the top contact andbottom contact of the switch. Even though a large out-of-plane movementoccurred in the simulation after release, there was predicted to beextremely low displacement at the contact region. This was furtherverified in a subsequent scanning electron microscope (SEM) image of thecompleted device (see below).

Example 2. Top Contact Design for Enhanced Strength

Due to the top contact being an important part of the switch thataffects reliability, efforts were made to design a structurally strongtop contact with a certain metal thickness. This was determined withinthe limit imposed by the deposition tool used (˜500 nm). The improveddesign shown in FIG. 11A, right side, employed a unique 3-dimensionalbowl-shaped top contact structure (111) that was estimated to have 3times the stiffness of the previously designed flat switch (“originaldesign” at left side of FIG. 11A) and 2.4-fold less maximum stress forthe same applied upward force. FIG. 11B shows an SEM image of afabricated 3D bowl-shaped switch. In practice, the bowl-shaped designhas proved to be extremely resilient and has provided switchingreliability of hundreds of cycles.

Example 3. Properties of an IR Plasmonic Absorber

The present PMR devices incorporate a plasmonic absorber that providesthe spectral selectivity required to distinguish between differentwavelengths of incoming IR or other radiation. The key goal of theplasmonic absorber design is to have a high absorption percentage withina narrow range of wavelengths, while reflecting other wavelengths.

The designed and fabricated plasmonic absorbers were measured to have aconsistently high absorption of >85% with a bandwidth of <380 nm. Thecore structure of the absorber consists of periodic square or crosspatterns of gold on a thin (˜200 nm) SiO₂ layer with a gold reflectorunderneath as shown in the inset of FIG. 12A. In FIG. 12A, thewavelength dependence of absorbance is shown for a patch patterncompared with a cross-shaped pattern.

Lithographic tuning to vary the size and/or periodicity of gold patternsof the plasmonic absorbers enables the tuning of the absorption band toa wavelength range of choice. Since one objective was to target emissionwavelengths of particular gases, different absorbers were fabricatedthat corresponded to emission peaks of different gases. FIG. 12B showsan FT-I R measurement of two absorbers tuned to two differentwavelengths, corresponding to absorption peaks of nitric oxide (NO) andcarbon monoxide (CO), using cross patterns. It is noted that the crosspattern provided about half the bandwidth of the square pattern as seenin FIG. 12A.

Example 4. Zero Power Detection of IR

IR radiation of a defined wavelength was detected while consuming 0watts of power. The device had a plasmonic absorber (square type) thatwas tuned to 5 μm wavelength with 90% absorption. By using a quantumcascade laser (QCL) as the radiation source, the device was able to turnON for an input IR power of 2.7 mW in air and had a contact gap of 2.3μm. The device has an absorber area of 120 μm×130 μm and was designed toturn ON for an input power of 1 μW in vacuum (10⁻⁵ Torr). FIG. 13 showsan SEM image of the tested device; the stress/temperature compensationis clearly seen to be effective. FIG. 14 shows a few cycles of theswitching when input IR power was increased and decreased. The devicewas biased at 0.05 V, implying an ON resistance of ˜3.3 kΩ, whichmatched the expected value. The ON power and the OFF power weredifferent as expected due to adhesion. This hysteresis, the gap size,and stiffness of the structure were used to effectively measure theadhesion force.

The stiffness of the structure was ˜50 nN/μm, which resulted in anextracted adhesion force of just 8.58 nN. This exceeded initialexpectations of ˜10 nN and therefore suggests that device sensitivitycan be <100 nW in vacuum. As a control, the same experiment was doneusing a similar device on a different chip, where the device had noplasmonic absorber pattern. As expected, IR radiation did not turn ONthe device even at >20 mW of power.

A reliability test also was performed by providing a constant input IRpower of 2.8 mW (just above threshold) which was chopped at 1 Hzfor >350 cycles. The device was completely functional at the end of theexperiment. FIG. 15 shows a part of the reliability test, demonstratingthe repeatability of the detection.

Example 5. Electrothermally-Actuated Switches

Since the detection mechanism involves the temperature rise of one head,an electric heater (121) can be integrated on the absorber side insteadof a plasmonic surface. This was experimentally demonstrated as anelectrothermally actuated switch both in air and vacuum. Incorporatingthe heater on the same design as shown previously, a turn ON power of0.932 μW was measured in vacuum (10⁻⁵ Torr) with a similar adhesion ofjust 10.9 nN. FIG. 16A shows one such heater-actuated design, and FIG.16B shows a typical ON-OFF cycle when increasing and decreasing power(voltage input) to the heater.

The same devices were actuated in air and were measured to turn ON for˜350 μW, as expected due to the much larger thermal conduction andlosses due to air. Thus, low power thermally actuated switches can bemade with low adhesion and immunity to temperature changes.

Example 6. Fabrication of an IR-Sensitive PMR

-   -   1. Silicon dioxide (SiO₂) was deposited on a blank 4-inch        silicon wafer as a 1.9 μm thick layer by plasma-enhanced        chemical vapor deposition (PECVD).    -   2. Next, the metallic base layer was deposited and patterned on        the cantilever heads. 10 nm titanium (Ti), 100 nm of platinum        (Pt) or gold (Au), and 10 nm titanium were deposited by e-beam        evaporation and patterned by LOR assisted lift-off. Ti layers        were used to promote the adhesion between adjacent materials.        (1st mask)    -   3. A second SiO₂ deposition was performed for the insulating        layer between metallic base layer and future patterned metallic        structures. A 100 nm thick SiO₂ layer was deposited by PECVD.    -   4. A second metal deposition and patterning was carried out for        the electrical routing (also to make resistive heaters for        electrothermal relays). 20 nm Ti and 20 nm Pt were deposited by        e-beam evaporation and patterned by LOR assisted lift-off. The        Pt layer was used to protect the Ti layer during the future        xenon difluoride (XeF₂) isotropic dry release of the device.        (2nd mask)    -   5. A third metal deposition and patterning was performed to form        the contact base (bottom contact). 5 nm Ti and 100 nm Pt were        deposited by e-beam evaporation and patterned by LOR assisted        lift-off. (3rd mask)    -   6. Aluminum (Al) deposition and patterning was performed next.        500 nm Al was deposited by sputtering and patterned by wet etch.        The wet etch only removed Al in the field (outside of bimaterial        legs), leaving the Al on the area of bimaterial legs untouched        for the future Al and SiO₂ co-etch. (4th mask)    -   7. Gold (Au) deposition and patterning was carried out to create        the reflector on cantilever heads and probing pads. 5 nm Ti and        150 nm Au were deposited by e-beam evaporation and patterned by        lift-off. (5th mask)    -   8. A second Au deposition and patterning was done with e-beam        lithography to create the plasmonic absorber. 5 nm Ti/50 nm        Au/10 nm Pt were deposited by e-beam evaporation and patterned        by e-beam lithography and LOR assisted lift-off. LOR used in the        lift-off process created a discontinuity in the sidewall of the        photoresist (PR) as shown in FIGS. 17A-17F, which helps in        achieving a flat edge of Au patches. The flatness of Au patches        on the edge effects the absorption properties of the plasmonic        absorber. The thin Pt layer here is for the protection of the Au        layer during the future XeF₂ release of the device. Steps        (a)-(f) as shown in FIGS. 17A-17F were as follows: (a) LOR 3A        was spin-coated at 4000 rpm and baked on a 200° C. hot plate for        5 minutes; (b) PMMA950K (diluted with thinner) was spin-coated        at 5000 rpm and baked on a 180° C. hot plate for 90 seconds; (c)        PMMA was exposed with electron beam, then the PMMA was developed        in MIBK:IPA (1:3) solution for 1 minute; (d) LOR was etched with        diluted AZ400K:DI (1:1) developer for 4-5 minutes, depending on        the feature size; (e) Ti/Au/Pt was deposited by e-beam        evaporation; and (f) sample was soaked in 1165 solution for        lift-off.    -   9. Al and SiO₂ co-etch was then performed. The Al and SiO₂ layer        was dry etched with the same mask to accurately define the size        of bimaterial legs and device shape. This is important for the        symmetry between the two pairs of bimaterial legs as the co-etch        guarantees a self-alignment between Al and SiO₂. (6th mask) The        pattern of Al and SiO₂ coherent dry etch (outer pattern shown in        FIG. 18 ) is intentionally designed to overlap with the pattern        of Al wet etch (inner pattern in FIG. 18 ). After the wet etch,        the longitudinal edge of Al will be further defined by Al dry        etch and followed by SiO₂ dry etch with a same mask (6th mask)        to ensure a perfect alignment between Al and SiO₂ layers on the        bimaterial legs.    -   10. Amorphous silicon (a-Si) deposition and patterning was        carried out for the sacrificial layer between contact base        (bottom contact) and contact tip (top contact). 500 nm a-Si was        deposited by sputtering and patterned by dry etch. (7th mask)    -   11. Shaping of the photoresist (PR) plug for trench        planarization was carried out. 1.3 μm positive PR was        spin-coated and patterned by photolithography inside the 2 μm        deep trench. The patterned PR plug was further hard baked at 150        Celsius for at least 5 mins and thinned down by oxygen plasma        etch. (8th mask) The profile of the plug during this process is        depicted in FIG. 19 .    -   12. Pt was deposited and patterned for the contact tips (top        contact). 10 nm Ti and 450 nm Pt were deposited by sputtering        and patterned by lift-off using 2.7 μm positive PR. It is        important to use sputtering for the deposition of Pt as it gives        a conformal coating across steps. The lift-off requires long        time soaking in PR stripper and gentle cleaning of the metal        residuals in order to achieve high yield of the bowl-shaped        contact tips. (9th mask)    -   13. Finally, the wafer was diced and the devices released by Si        isotropic etch with XeF₂.

The details of contact tip fabrication are shown in FIGS. 20A-20F. Thestructures shown are as follows: (a) cross section of two heads afterfabrication process step 8; (b) step 9, Al dry etch and SiO₂ co-etch;(c) step 10, amorphous silicon (a-Si) deposition and patterning for thesacrificial layer; (d) step 11, shaping of photoresist (PR) plug fortrench planarization; (e) step 12, Pt deposition and patterning for thecontact tips; (f) device release.

As used herein, “consisting essentially of” allows the inclusion ofmaterials or steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising”, particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with “consisting essentially of” or “consisting of”.

While the present invention has been described in conjunction withcertain preferred embodiments, one of ordinary skill, after reading theforegoing specification, will be able to effect various changes,substitutions of equivalents, and other alterations to the compositionsand methods set forth herein.

The invention claimed is:
 1. A device comprising: a substrate, at leastone electrical contact disposed on a surface of the substrate; acantilever disposed on the substrate, the cantilever comprising a head,the head including a contact element disposed at a free end of thecantilever for movement into and out of electrical contact with the atleast one electrical contact disposed on the surface of the substrate,and the contact element comprises a metal bowl-shaped structurecomprising: a contact surface facing the at least one electrical contactdisposed on the surface of the substrate, a central portion comprisingan interior surface facing in a direction away from the at least onecontact, and a peripheral portion extending from the interior surface inthe direction facing away from the at least one electrical contact,wherein the central portion and the peripheral portion define aninterior space of the bowl-shaped structure open in the direction facingaway from the at least one electrical contact; wherein the cantileverfurther comprises an inner pair of temperature-sensitive bimateriallegs, and an outer pair of temperature-sensitive bimaterial legs, theinner pair of legs attached to opposite sides of the head, the outerpair of legs attached to the surface of the substrate and disposedadjacent to the inner pair of legs forming first and second sets ofinner and outer legs, the first and second sets of legs disposedsymmetrically on opposite sides of the head; and a first thermalisolation region connects the inner and outer legs of the first set oflegs, and a second thermal isolation region connects the inner and outerlegs of the second set of legs; and wherein the device is amicroelectromechanical device, and the head has a microscale length andwidth and a nanoscale or microscale thickness.
 2. The device of claim 1,wherein the central portion has a thickness of approximately 500 nm. 3.The device of claim 1, wherein the contact element includes an extendingcontact tip, the contact tip having a width of about 4 μm.
 4. The deviceof claim 1, wherein the contact element has a stiffness greater than astiffness of the cantilever.
 5. The device of claim 1, wherein thecontact element has a stiffness greater than 0.05 N/m.
 6. The device ofclaim 1, wherein the contact element has a stiffness greater than 40N/m.
 7. The device of claim 1, wherein the contact element has astiffness of about 0.05 N/m to about 1000 N/m.
 8. The device of claim 1,wherein the contact element has a stiffness of about 40 N/m to about1000 s of N/m.
 9. The device of claim 1, wherein the head furthercomprises a metallic layer on the surface facing the surface of thesubstrate and an insulating layer on the metallic layer.
 10. The deviceof claim 1, wherein the cantilever comprises a pair of bimaterial legs,each of the bimaterial legs comprises a stack of at least two materialshaving different thermal expansion coefficients, the legs providingcompensation for ambient temperature changes.
 11. The device of claim 1,wherein the at least one electrical contact comprises a sourceelectrical contact or a drain electrical contact, and further comprisingan additional electrical contact comprising the other of the sourceelectrical contact and the drain electrical contract disposed on thesurface of the substrate, the at least one electrical contact and theadditional electrical contact separated by a gap; and the absorption ofthe electromagnetic radiation causes the contact element to move towardthe source electrical contact and/or the drain electrical contact. 12.The device of claim 1, wherein the head further comprises a plasmonicabsorber that absorbs electromagnetic radiation within a spectral bandselected for detection, and the absorption of such electromagneticradiation causes the contact element to move toward the electricalcontact disposed on the surface of the substrate.